"Salmon have a “feed conversion ratio” of roughly one to one; that means they produce about one kilogram of body mass for every kilogram of feed consumed. Cows, on the other hand, require 8 to 12 kilograms of feed to gain a kilogram of mass."
https://spectrum.ieee.org/aquaculture
https://spectrum.ieee.org/aquaculture
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Cows feeding on grass + herbs versus salmon feeding on fish meal consisting of protein + fat.
All predators have lower conversion rates than herbivores!